The present invention relates generally to the field of xylenes production. More particularly, it concerns methods for converting toluene to xylenes using catalyst systems capable of producing p-xylene in high yield.
Methylation of toluene with methanol is known in the art as a useful technique for the formation of xylenes. The reaction has the following general formula (Formula I): 
wherein x, y, and z are each between 0 and 1, inclusive, and x+y+z=1.
From top to bottom on the right of Formula I are ortho-xylene (o-xylene, 1,2-dimethylbenzene), meta-xylene (m-xylene, 1,3-dimethylbenzene), and para-xylene (p-xylene, 1,4-dimethylbenzene). Of the three, p-xylene is particularly useful in making, either directly or by substitution at the methylene moieties, straight-chain polymers such as polyethylene terephthalate (PET). Generally, however, the methylation of toluene produces all three of the xylene isomers shown in Formula I. Although the separation of p-xylene from a raw product mixture containing the other xylene isomers is possible, it will be apparent that techniques for maximizing the fraction of the raw product mixture defined by p-xylene (i.e., increasing z in Formula I) are beneficial.